This application claims priority form Japanese Patent Application Serial No. 2000-166708, filed Jun. 2, 2000.
1. Field of the Invention
This invention relates to a holding apparatus for affixing an object by attractive forces. In further detail, this invention relates to a holding apparatus for clamping a plate-shaped object such as silicon wafer, printing substrate, glass substrate and the like by attraction means such as electrostatic chucking and vacuum chucking.
2. Background Technology
Conventionally, as a technology of holding a plate-shaped specimen in place, holding apparatuses for affixing an object in place by attractive forces have been known, in the form of vacuum chucking, electrostatic chucking and the like. Such attraction holding apparatuses are used, for example, for transporting plate-shaped specimens and for clamping such plate-shaped specimens during processing such as light exposure, film deposition, micro-fabrication, cleaning and dicing and the like.
In such an attraction holding apparatus, on the opposing surface of the attraction holding apparatus that faces the specimen, latticed slits or protrusions or holes are formed, so that surfaces contacting the specimen and surfaces not contacting the specimen are created. In so doing, in the case of the electrostatic holding, creation of non-contacting surfaces on the plane opposite to the specimen enables to facilitate detachability after the applied voltage is removed, or, in the case of vacuum chucking, it enables to maintain a vacuum between the attachment surface and the specimen by connecting the space between the lower surface of the specimen and the non-contacting surface to an evacuation apparatus. Further, in either type of attraction holding methods, the presence of the non-contacting surface prevents intrusion of dust and foreign particles in-between the contacting surface and the specimen.
Also, when a plate-shaped specimen is to be subjected to processing steps such as etching in a plasma atmosphere, it is necessary to perform such a process under a reduced pressure, i.e., in an environment to promote the formation of plasma. When processing the plate-shaped specimen, for example, etching of semiconductor wafers in a plasma atmosphere, the surface of the semiconductor wafers are heated to high temperatures due to plasma heating. Then, as the surface heats up, problems such as bursting of the resist film formed on the surface may occur. Also, if the temperature distribution on the specimen surface is non-uniform, resulting chemical processing would be non-uniform. Therefore, to provide uniform chemical processing of the plate-shaped specimen, attraction holding apparatus based on electrostatic chucking is designed so as to cool the surface temperature of the plate-shaped specimen uniformly by flowing a cooling gas such as helium through the space between the lower surface of the plate-shaped specimen and the non-contacting surface.
In this case, higher cooling effect is obtained by increasing the surface area of the non-contacting surface as much as possible. However, if the area of the contacting surface is made too small in an attempt to increase the surface area of the non-contacting surface, attraction force would be reduced. Therefore, attempts are made to obtain sufficient attraction force even with small area of attachment surfaces, such as increasing the degree of smoothness of the contacting surface. For example, a Japanese Patent Application, First Publication Hei 7-153825, disclose an electrostatic chuck such that the specific resistance of the dielectric layer is less than 109xcexa9m, Rmax (maximum height) of the upper surface of a plurality of protrusions that serve as the attachment surface is less than 2.0 m, or Ra (center line average roughness) is less than 0.25 xcexcm, and an area ratio of the total surface area of the upper surface of the protrusions to the upper surface area of the dielectric layer is more than 1% and less than 10%.
Also known is a holding apparatus that can perform clamping and heating of the plate-shaped specimen concurrently. This holding apparatus is constructed so that the plate-shaped specimen is heated by flowing electrical current through an internally housed heater so as to affix the plate-shaped specimen to the attachment surface of the holding apparatus while heating the attachment surface so as to heat the plate-shaped specimen.
In such a holding apparatus also, it is required to produce a uniform surface temperature to provide uniform chemical processing of the plate-shaped specimen. For this reason, a gas such as helium is flowed in the space between the lower surface of the plate-shaped specimen and the non-attaching surface so as to distribute the heating effect of the heater over the entire surface of the plate-shaped specimen.
As described above, to form non-attaching surfaces opposing the specimen on the holding apparatus serves not only to improve detachability, secure a suction path for providing vacuum suction and prevent adhesion of dust particles, but also to play an important role of providing uniform chemical processing to the plate-shaped specimen by equalizing the temperature of the plate-shaped specimen.
However, in the conventional attraction holding apparatus having non-attaching surfaces described above has not always produced uniform temperature in terms of microscopic regions of the plate-shaped specimen. It is because the contacting surface conducts heat much more readily relative to the non-attaching surfaces so that, in the case of heating the plate-shaped specimen using a heater, those regions in contact with the attachment surface tend to reach higher temperatures than those not in contact, and, in the case of heating the plate-shaped specimen with plasma and the like, non-contact regions tend to reach higher temperatures because they provide no place for the heat to drain away.
Especially, if it is not able to introduce a gas such as helium, adverse effects caused by non-uniform temperature due to reasons cited above have been noted. Even in those cases that allow a gas such as helium to be introduced, the effect of equalizing the temperature due to helium gas is difficult to reach, from microscopic viewpoint, to the contacted portions of the plate-shaped specimen that are touching the attaching surface, and it has been difficult to obtain sufficient temperature uniformity. Here, although it may be expected that increasing the pressure of the gas such as helium would increase the temperature homogenizing effect, but excessive gas pressure is not appropriate because it introduces problems such as breakage of the plate-shaped specimen.
To resolve microscopic non-uniformity in temperature distribution on the surface of the plate-shaped specimen described above, it is effective to reduce the area of attachment as much as possible, and for example, when numerous protrusions are erected, it is effective to reduce the upper surface area of each protrusion as much as possible.
However, if the area of attachment surface is reduced, resistance to chemical attack is lowered and the protrusions become vulnerable to damage, and further, peripheries of the attachment surface becomes vulnerable to wear due to contact with the plate-shaped specimen.
This invention is provided in view of the background information described above, and an object is to provide a holding apparatus that improves the uniformity of surface temperatures in microscopic regions of a plate-shaped specimen while exhibiting sufficient holding strength and detachability after impressed voltage is removed (in the case of electrostatic chucking), without causing degradation in the chemical and mechanical properties at the attachment surfaces contacting the specimen, such as the upper surfaces of the protrusions, and without having to introduce excessive amount of gas.
It has been found, after intensive studies, that the above problem can be resolved effectively by modifying the apex shape of the protrusions that traditionally have been made into a simple flat apex, to lead to completion of this invention.
That is, the holding apparatus of this invention comprises a flat base, and a plurality of protrusions erected in the attachment region in the flat base, for attraction fixation of a plate-shaped specimen on the plurality of protrusions, wherein each upper surface of the protrusions is comprised by a specimen holding surface and a concavity.
According to this invention, because a portion of the upper surface of each protrusion is made non-contacting, so that difference in the manner of heat conduction through the portions of the attachment region that have no protrusions (referred to as grooves hereinbelow) and through the overall upper surfaces can be minimized. And therefore, it enables to homogenize the temperature of the plate-shaped specimen even from a microscopic viewpoint. In this case, although the concavity may be formed in any shape in any location, it is preferable that it is formed in a location surrounded by the specimen holding surface. In so doing, it becomes more difficult to damage the specimen holding surface even if it touches the specimen, and furthermore, heat transfer is lower in closed spaces, and therefore, it is possible to further homogenize the temperature of the specimen even from a microscopic viewpoint.
Also, a width of the specimen holding surface is not more than twice a thickness of a plate-shaped specimen to be clamped. Here, the width of the specimen holding surface means a spacing between the outer periphery of the protrusion upper surface and the outer periphery of the concavity surrounded by the specimen holding surface. By so doing, the difference in the manner of heat conduction through the grooves and through the protrusion upper surfaces can be further minimized, so that it is possible to further homogenize the specimen temperature even from a microscopic viewpoint.
Also, it is preferable that an area ratio of a total area of the specimen holding surfaces to a total area of upper surfaces of the protrusions be in a range of 10-90%. By so doing, sufficient adhesion strength and uniformity of specimen temperature can be secured at the same time. That is, if the area ratio is less than 10%, sufficient adhesion strength cannot be obtained, and if the area ratio exceeds 90%, it means, in effect, that the concavities are not formed on the protrusions, so that it becomes difficult to homogenize the specimen temperature.
Another holding apparatus of this invention is provided with a flat base, and a plurality of protrusions erected in the attachment region in the flat base, for adhesion fixation of a plate-shaped specimen on the plurality of protrusions, wherein each upper surface of the protrusion is comprised by a planar region having a center line average roughness of not more than 0.5 xcexcm and a rough region having a center line average roughness of not less than 0.5 xcexcm.
According to this invention, roughening portions of the upper surface of each protrusion enables to lower the degree of intimate contact between the roughened portions and the plate-shaped specimen, so that the difference in the manner of heat conduction through the grooves and through the protrusion upper surfaces can be minimized. And therefore, it is possible to homogenize the specimen temperature even from a microscopic viewpoint.
Here, the reason for specifying the center line average roughness of the planar regions to be less than 0.5 xcexcm and the center line average roughness of the rough regions to be more than 0.5 xcexcm is that, the thermal conductivity behaviors differ significantly through the planar regions having the center line average roughness of less than 0.5 xcexcm and through the rough regions having the center line average roughness of more than 0.5 xcexcm, across the boundary value of 0.5 xcexcm of the center line average roughness. And, it is possible to secure sufficient adhesion strength by producing the center line average roughness of less than 0.5 xcexcm in the planar regions.
In this case, although the rough region can be formed in any location of the upper surface of the protrusions, it is preferable that the rough region be formed in a location surrounded by the planar region. In so doing, it becomes more difficult to damage the specimen holding surface even if it touches the specimen, and furthermore, heat transfer is lower in closed spaces, and therefore, it is possible to further homogenize the temperature of the specimen even from a microscopic viewpoint.
Also, it is preferable that a width of the planar region is not more than twice a thickness of a plate-shaped specimen to be clamped. Here, the width of the specimen holding surface means a spacing between the outer periphery of the protrusion upper surface and the outer periphery of the rough region surrounded by the planar region. By so doing, the difference in the manner of heat conduction through the grooves and through the overall upper surfaces of the protrusions can be further minimized, so that it is possible to further homogenize the specimen temperature even from a microscopic viewpoint.
Also, it is preferable that an area ratio of a total area of the specimen holding surfaces to a total area of upper surfaces of the protrusions be in a range of 10-90%. In so doing, it is possible to secure sufficient adhesion strength and the temperature uniformity of the plate-shaped specimen at the same time. That is, if the area ratio is less than 10%, necessary adhesion strength cannot be secured while, if the area ratio exceeds 90%, it means in effect that rough regions have not been formed, and it becomes difficult to homogenize the temperature of the plate-shaped specimen.
It has thus been possible to provide a holding apparatus that enables to improve the uniformity of surface temperatures in microscopic regions of a plate-shaped specimen while exhibiting sufficient holding strength and detachability after the impressed voltage is removed (in the case of electrostatic chucking), without causing degradation in the chemical and mechanical properties at the surfaces contacting the specimen, such as the upper surfaces of the protrusions, and without having to introduce excessive amount of gas.